Studies are being done to investigate the electrophysiological characteristics of epithelial tissues, namely isolated perfused renal tubules, frog skin and toad urinary bladder. In particular, assessments are being made of the transepithelial driving forces for active transport in studies of the I-V relationships of these tissues and the routes of active and passive transport. In addition, the studies are designed to permit an analysis of the effects on the mechanisms of transport and their modifications in response to drugs and hormones that are known to alter the electrophysiological characteristics and rates of ion transport. In particular, microelectrode studies of the frog skin will permit an analysis of the effects at the outer and inner barriers of the transporting cells will special regard to the voltages and resistances of these barriers and the process of electrical rectification at the outer barrier.